Extruded matte anodised sections are made in large quantities for architectural and other use. The aluminium alloys used are 6000 series alloys in the Aluminum Association Register. This invention is concerned with the compositions of the alloys.
The standard production method involves extruding a billet of the chosen alloy, subjecting the extruded section to an alkaline etch, and anodising the resulting matte surface. Studies on the effect of composition on matte etching response have been published and each shows the importance of microstructural features on surface quality. Typically it has been demonstrated that constituent particles, dispersoids and ageing precipitates influence the surface evolution and final appearance. The effects of solid solution content, with the exception of Zn additions, have largely been ignored.
However, there are three important aspects of the aluminium extrudate etching process. Firstly, the extrudate surface finish from the press influences the uniformity of appearance, with a good finish requiring less metal removal to achieve acceptable quality. Secondly, the metal removal rate controls the amount of effluent that has to be controlled and disposed of; hence there are significant environmental benefits to reducing the amount of metal removed for an acceptable finish. Thirdly, the final matte appearance of the extrudate is critical and this means both low gloss (i.e. low brightness) and uniformity.
The benefit of high Fe content to matte finish has been recognised for some time and this has traditionally been used by the aluminium and metal finishing industries. The drawbacks of a high Fe content are: 1) the extrudate surface roughness increases making the product incompatible with others being produced on the same press where mill finish is critical and 2) a higher metal removal is needed to attain a uniform etched finish.
This invention results from the inventors"" discovery that control of the Cu content, and to a lesser degree also the Cr content, of the alloy can have a beneficial effect. The invention thus provides a population of billets resulting from more than one cast of metal having a specification such that every billet has a composition (in wt %):
A cast is defined as the process of converting a body of molten metal into a plurality of billetsxe2x80x94often several hundred billetsxe2x80x94of solid metal. The body of molten metal has a composition which is controlled to fall within a predetermined specification and which is generally given to the purchaser or user of the billet. The specification is maintained for more than one cast, generally for a whole series of casts. In the present invention the specification (which is not defined herein) is such that every billet has a composition within the ranges given above. A population of billets is an unspecified number, usually at least 50 and generally much more than 100, of billets resulting from more than one cast, usually a series of at least 5 and often more than 100 casts, of metal within the specification. A population according to the invention would not be expected to contain any billet having a composition outside the stated range.
Extruded sections are made by extruding billets taken from the population. Preferably the extruded sections are subjected to an alkaline etch and are then anodised. The invention also includes extruded sections so made.
The above alloys are within the 6000 series of the Aluminum Association classification and are related to M6060 and AA6063 generally used to make extruded matt anodised sections.
Mg and Si combine to form dispersed Mg2Si particles which contribute to dispersion strengthening of the extruded sections. If Mg or Si concentrations fall below the stated ranges, then extruded sections may not achieve desired mechanical properties in the T5 or T6 temper. When the extruded sections are subjected to alkaline etch, the Mg2Si particles are preferentially dissolved. To some extent, this is advantageous in enhancing the desired mattening effect. But if the Mg and Si contents are too high, problems may arise with regard to ease of extrusion and surface quality obtainable. In some circumstances it is preferable that the Mg content be in the range 0.35-0.45%.
Fe is a preferred constituent of the alloy, partly because it contributes to the desired mattening effect and partly because alloys containing no Fe are much more expensive. When the Fe content is too high, problems arise as discussed above.
Mn is beneficial to the desired etch response and helps to counteract Fe by reducing pitting activity. Zn is notorious for the production of a bright spangled appearance. At high concentrations, Ti can give rise to streaking.
The level of Cu is controlled to be less than 0.015%, preferably less than 0.010%. As the experimental data below show, higher levels of Cu have a detrimental effect on matte finish and increase the rate of metal removal during etching. These very low Cu levels cannot be consistently achieved without positive and deliberate control over alloy composition.
The level of Cr is kept below 0.10% as is conventional. But an addition of Cr at a level of 0.03-0.09% may be made. As the experimental data below show, Cr at these levels enhances the matte response to etching but without increasing the metal removal rate.
The balance of the alloy is aluminium of commercial purity. This will normally be primary Al from a smelter, since it would not be easy to achieve tight compositional control of secondary Al from scrap. The invention is concerned with commercial scale production, and not with laboratory experiments using high purity samples.
In performing the invention method, an Al alloy of chosen composition is cast into a billet which is optionally homogenised and extruded into a section which is cooled. Homogenising conditions do not appear to have any material effect on the development of a matte surface. The extruded section may be cooled in still air or more preferably by forced air cooling or quenching.
The extruded section is preferably aged e.g. to T5 or T6 temper. This may be effected by heating the section at 150-200xc2x0 C. for a time to develop peak strength. A preferred regime is 170-185xc2x0 C. for 5-6 hours. Ageing has a material effect on mattness. It is believed that ageing grows Mg2Si particles and that these dissolve during alkaline etch to give a matt finish.
The extruded section is subjected to alkaline etching to develop a matte surface. Mention may be made of two commercially available etch systems:
Long-life etch is mainly used in Europe and North America, and involves treatment for 5-20 minutes with a solution of
100 g/l NaOH
100-160 g/l Al ion
30-50 g/l sequesterant e.g. Na gluconate or Na heptonate
at 50-75xc2x0 C. This typically results in 100 g/m2 metal removal.
Recovery etch is mainly used in Japan and Canada, and involves treatment for 1-10 minutes with a solution of
30 g/l NaOH
50 g/l Al ion
at 50-75xc2x0 C. The weaker etch solution and shorter etch time results in a lower level of metal removal.
After etching, the extruded section has a matte surface. Although mattness is generally understood as the opposite of glossiness, its measurement is somewhat problematic and does vary substantially depending on the nature of the surface and of the treatment it is subjected to. Mattness may be measured by the test in BS 6161 at 60xc2x0. As a rough guideline, an Al surface that has been subjected to a long life etch may be regarded as matte if it has a gloss value below about 100; and an Al surface that has been subjected to a recovery etch may be regarded as matte if it has a gloss value below about 150.
Then the extruded and etched section is anodised under conditions which may be conventional and which form no part of this invention.
The ability to control etching response is important to ingot producers, extruders and finishers. The knowledge that two key parameters (Cu and Cr) have such a large influence is surprising. Armed with this knowledge, an ingot producer can control performance downstream when other factors are beyond its control.